1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which carries out printing by using a liquid ejection head having pressure generating elements corresponding to a plurality of ejection ports (nozzles), and to drive control technology for a liquid ejection head suitable for the image forming apparatus.
2. Description of the Related Art
In general, in an inkjet type recording apparatus (inkjet printer), ink droplets are ejected at prescribed timings from the nozzles of the recording head, on the basis of the dot pattern data (also called “dot data” or “print data”) expanded from image data for printing which has been input from a host computer. Printing is carried out by means of these ink droplets landing on and adhering to a print recording medium, such as a piece of recording paper.
As a system of the recording head, for example, a system where ink droplets are ejected by causing change in the volume of pressure chambers (pressure generating chambers) connected to nozzle apertures, is known. In a recording head of this kind, a diaphragm which is elastically deformable in the outward direction is formed on a portion of the circumferential walls which demarcate the pressure chambers, and the volume of the pressure chambers is changed by causing this diaphragm to vibrate by means of pressure generating elements typified by piezoelectric elements.
Generally, a plurality of nozzle apertures are formed in the recording head, and a pressure chamber and a piezoelectric element are provided for each one of the plurality of nozzle apertures. All of the piezoelectric elements are electrically connected in parallel between a common power supply and ground wire, and a switching element is electrically connected in series with each of the piezoelectric elements. A signal (which is also referred to as “drive waveform” or “waveform”) for driving the piezoelectric elements is generated by a drive waveform generating circuit, and it is selectively distributed and supplied to the piezoelectric elements via the power supply line and the switching elements.
More specifically, if a prescribed switching element, such as a switching array or analogue switch is selected and switched on according to the print data, for example, then a drive waveform is applied to a piezoelectric element via the power supply line, and an ink droplet is ejected from a prescribed nozzle aperture corresponding to the piezoelectric element to which the drive waveform has been applied.
In an inkjet recording apparatus which uses piezoelectric elements as described above, a common drive circuit system is generally adopted. In the common drive circuit system, one common drive waveform formed by combining a plurality of drive waveform elements for ejecting a plurality of types of ink droplets of different ink volumes (for example, a large dots, a medium dot, and a small dot) is used, and the required waveform portion is selectively applied to each of the piezoelectric elements by means of switches (see Japanese Patent Application Publication Nos. 2002-154207 and 2000-37867). In this system, there is no need to prepare drive waveform generating circuits individually for each of the piezoelectric elements because the common drive waveform is simultaneously applied to a plurality of piezoelectric elements. Therefore, this system has benefit in that the number of analogue circuits for high-voltage and high-precision, and the number of wires, can be reduced.
On the other hand, in view of the object of increasing printing speed and the like, in recent years, array type and line type printers have been proposed, in which a very large number of nozzles are prepared, ink being simultaneously ejected from the large number of nozzles in such a manner that printing is carried out quickly. In an array type or line type recording head having a large number of nozzles, if the common drive circuit system described above is used without modification, then a large number of piezoelectric elements are simultaneously driven according to drive waveforms output from a single drive circuit, the drive waveform is distorted by the load fluctuation, ejection errors may arise, and consequently non-uniformities in image quality may occur.
Various methods have been proposed in view of these. For example, a method where a dummy load element having an electrostatic capacitance (for example, several hundred nanofarads (nF) for approximately 1000 elements (nozzles)), which exceeds the electrostatic load capacitance of the piezoelectric elements (“several hundred picofarads (pF) per element” multiplied by “the number of elements”) is provided (connected) either before or after the power supply line (signal wire), a method where various feed back controls based on a circuit (hardware) are applied to the drive circuit, a method where various feed back controls based on a software are applied to the drive circuit, and the like.
FIG. 24 shows the principal composition of the circuit required for head driving on the basis of the related art technology. As shown in FIG. 24, a ceramic capacitor 502 forming a dummy load element is provided before the flexible substrate 500 forming the power supply line, and a ceramic capacitor 504 forming a dummy load element is provided after the flexible substrate 500. At least one of the ceramic capacitors 502 and 504 should be connected to the flexible substrate 500.
Furthermore, as an example of providing to a signal line a dummy load element having a larger capacitance than the load capacitance of the piezoelectric element, there is a method in which each head is driven by selecting and adjusting the dummy load element for each head, in order to operate a plurality of heads having piezoelectric elements with different specifications in the same apparatus, as described in Japanese Patent Application Publication No. 2004-122120. The time constant τ which is a factor that flattens the drive signal, can be expressed as τ=C×R, on the basis of the electrostatic capacitance component C of the piezoelectric element, and the resistance component R, such as the wiring resistance, on resistance of analog switch. A method where the electrostatic capacitance component C and the resistance component R are adjusted in such a manner that this time constant τ is uniform, is known.
Furthermore, as an example of applying a circuit-based feedback to the drive circuit, there is a method where the feedback method is adjusted and a dummy load element is prepared in the piezoelectric element connection sections, in such a manner that variation in the on-resistance of the analog switches can also be adjusted, as described in Japanese Patent Application Publication No. 2002-59543. As an example of applying control-based feedback to the drive circuit, there is a method where the capacitance value of the required load is monitored, a dummy load element is selected on the basis of the monitoring results, and the capacitance of the dummy load element is determined in such a manner that the sum of the capacitance load and the dummy load is constant, as described in Japanese Patent Application Publication No. 11-320872.
However, in a print head having a plurality of nozzles exceeding 1000 nozzles, such as an array or line type of print head which ejects ink substantially simultaneously from the large number of nozzles in order to perform high-speed recording, a plurality of drive circuits constituted by common drive circuit systems need to be prepared. In this system, it is necessary to provide each of the drive circuits with a dummy load element, and hence the circuits may become large in size and the installation may become difficult. In addition to this, it is necessary to adjust the drive circuits in such a manner that the standards of the electrostatic capacitance value of the drive circuits are different from each other. For example, if a situation occurs where the electrostatic capacitance relating to ejection is low and the dummy load is high in a certain drive circuit and the electrostatic capacitance relating to ejection is high and the dummy load is low in another drive circuit, then some dummy loads are redundantly driven and there is an insufficiency in the electrostatic capacitance of the dummy loads used. As a result of that, power is wastefully consumed in the drive circuits (the power consumption in the drive circuits gets increase).
Specifically, if not only a large number of piezoelectric elements but also dummy load elements are simultaneously driven, then the drive circuit instantaneously consumes a large amount of power. Therefore, a voltage drop may occur in the power supply. In order to avoid the voltage drop of this kind in the power supply and to drive the piezoelectric elements reliably, it is necessary to provide a power supply apparatus having a large power supply capacity.
Furthermore, if a large amount of energy is consumed by the drive circuit and the drive energy is insufficient, then ink ejection may become unstable, and quality deterioration may arise in the recorded image.